1. Field of Invention
The present invention relates in general to a plane light source, and more particularly, to a backlight module for a liquid crystal display.
2. Related Art
As the performance of computers has been greatly improved, and the technologies of Internet and multi-medium have been highly developed, analog transmission of image information has been replaced by digital transmission. To compromise the modern life style, the video or image device is made thinner and lighter. Although the conventional cathode ray tube (CRT) display has the persistent advantage, the structure of the internal electronic cavity results in a large volume of the display and causes great radiation to hurt human's eyes. Therefore, the flat panel displays (FPDs) developed based on the optoelectronic and semiconductor technologies such as liquid crystal displays (LCDs), organic electro-luminescent displays (OELDs) or plasma display panels (PDPs) have become the mainstream display products.
The liquid crystal displays are categorized into three different types based on the light sources used thereby, including the reflective liquid crystal displays, the transmissive liquid crystal displays, and the trans-reflective liquid crystal displays. The transmissive or the trans-reflective liquid crystal displays are constructed by a liquid crystal panel and a backlight module. The liquid crystal panel includes two transparent substrates and a layer of liquid crystal sandwiched between two transparent substrates. The backlight module is used to provide a plane light source to the liquid crystal panel, so as to achieve the display effect. Generally speaking, the backlight module can be divided into direct-type or edge-lit type, where the direct-type backlight module is applied to the liquid crystal panel with larger size.
FIG. 1 illustrates a cross sectional view of a conventional backlight module. Referring to FIG. 1, the conventional backlight module 100 is a direct-type backlight module that includes a light box 100, a plurality of cold cathode fluorescence lamps (CCFL) 120, a diffusion plate 130, and an optical film 140. The cold cathode fluorescence lamps 120 are disposed in the light box 110. The light emitted from the cold cathode fluorescence lamps 120 is diverged within the light box 110 and transmit through the diffusion plate 130 and optical film 140, so as to provide a plane light source.
In the above backlight module 100, the cold cathode fluorescent lamps 120 are distributed in a way that a brighter light is resulted at the position above the cold cathode fluorescent lamps 120. Therefore, the uniformity of the plane light source is normally poor.
FIG. 2 shows the light divergence of the backlight module. As shown, cold cathode fluorescent lamps 120 are scattered in the light box 100. If the distance between the cold cathode fluorescent lamps 120 and the diffusion plate 130 is insufficient (such as d2) for the light beams to spread, the light 122 above the diffusion plate 130 is more concentrated to worsen the uniformity of the plane light source. Therefore, the distance between the bottom of the light box 110 and the diffusion plate 130 has to be as long as d1 to allow the light beam to spread sufficiently, so as to maintain the uniformity. In other words, the overall thickness of the backlight module 100 cannot be reduced.